We're told (and we can confirm) that , so is a linear combination of the other two vectors.
This means <em>H</em> is sufficiently spanned by ; no need for the third vector.
But this also means we can write either as a linear combination of , and as a lin. com. of . So any set of these three vectors taken two at a time will span the subspace <em>H</em>. Hence all of b, c, and d are acceptable.